In the manufacture of liquid crystal cells, two glass plates are joined together with a layer of a liquid crystal material sandwiched between them. The glass substrates have conductive films thereon (at least one must be transparent, such as an ITO film) that can be connected to a source of power to change the orientation of the liquid crystal material. Various areas of the liquid crystal cell can be accessed by proper patterning of the conductive films. More recently, thin film transistors have been used to separately address areas of the liquid crystal cell at fast rates. Such liquid crystal cells are useful for active matrix displays such as TV and computer monitors.
As the requirements for resolution of liquid crystal monitors has increased, it has become desirable to separately address a plurality of areas of the liquid crystal cell, called pixels. Since about 1,000,000 pixels are present in modern displays, at least the same number of transistors must be formed on the glass plates so that each pixel can be separately addressed.
Different types of thin film transistors are in current use, but most require deposition of a gate dielectric layer over a patterned gate metal with an amorphous silicon layer thereover. The gate dielectric layer can be made from various combinations of silicon oxide, silicon nitride and metal oxide layers. For example, the metal contact can be covered with one or more layers of tantalum oxide, aluminum oxide or silicon oxide, and a final layer of high quality gate dielectric. Silicon nitride is usually the gate dielectric layer; it can also be used alone. Metal contacts are deposited thereafter over the amorphous silicon films, which can have a doped amorphous silicon layer thereover to improve contact between the amorphous silicon and the overlying metal contacts. A second silicon nitride film is deposited over the amorphous silicon layer as an etch stop in some transistor designs.
Up till now, thin silicon nitride films have had adequate quality for use in thin film transistors; however, the deposition rate has been quite slow. Thus it would be highly desirable to be able to deposit silicon nitride films at increased deposition rates while maintaining high quality films.